The present invention relates to the electrolytic production of high purity alkali metal hydroxide solutions. The alkali metal hydroxides of the present invention are produced along with halides utilizing membrane electrolytic cells by the passage of an electric current through an alkali metal halide solution.
Electrolytic cells that are commonly employed commercially for the conversion of alkali metal halides into alkali metal hydroxides and halides may be considered to fall into the following general types: (1) diaphragm, (2) mercury, and (3) membrane cells.
Diaphragm cells utilize one or more diaphragms permeable to the flow of electrolyte solution but impervious to the flow of gas bubbles. The diaphragm separates the cell into two or more compartments. Upon imposition of a decomposing current, halide gas is given off at the anode, and hydrogen gas and alkali metal hydroxide are formed at the cathode. Although the diaphragm cell achieves relatively high production per unit floor space, at low energy requirements and at generally high current efficiency, the alkali metal hydroxide product, or cell liquor, from the catholyte compartment is both dilute and impure. The product may typically contain about 12 percent by weight of alkali metal hydroxide along with about 12 percent by weight of the original, unreacted, alkali metal chloride. In order to obtain a commercial or salable product, the cell liquor must be concentrated and purified. Generally, this is accomplished by evaporation. Typically, the product from the evaporators is about 50 percent by weight alkali metal hydroxide containing about 1 percent by weight alkali metal chloride.
Mercury cells typically utilize a moving or flowing bed of mercury as the cathode and produce an alkali metal amalgam on the mercury cathode. Halide gas is produced at the anode. The amalgam is withdrawn from the cell and treated with water to produce a high purity alkali metal hydroxide. Although mercury cell installations have a high initial capital investment, undesirable ratio of floor space per unit of product, relatively poor power efficiencies, and negative ecological considerations, the purity of the alkali metal hydroxide product is an inducement to its use. Typically, the alkali metal hydroxide product contains less than 0.05 percent by weight of contaminating foreign anions.
Membrane cells utilize one or more membranes or barriers separating the catholyte and the anolyte compartments. The membranes are permselective, that is, they are selectively permeable to either anions or cations. Generally, the permselective membranes utilized are cationically permselective. In membrane cells employing a single membrane, the membrane may be porous or non-porous. In membrane cells employing two or more membranes, porous membranes are generally utilized closest to the anode, and non-porous membranes are generally utilized closest to the cathode. The catholyte product of the membrane cell is a relatively high purity alkali metal hydroxide. Examples of membrane cells are described in U.S. Pat. Nos. 3,017,338; 3,135,673; 3,222,267; 3,496,077; 3,654,104; 3,899,403; 3,954,579; and 3,959,085. The catholyte product, or cell liquor, from a membrane cell is purer and of a higher concentration than the product of a diaphragm cell.
It has been the objective, but not the result, for diaphragm and membrane cells to commercially produce "rayon grade" alkali metal hydroxide, that is, a product having a contamination of less than about 0.5 percent of the original salt. Diaphragm cells have not been able to produce such a product directly, because anions of the original salt freely migrate into the catholyte compartment of the cell. Membrane cells have the capability to produce a high purity alkali metal hydroxide product. A problem encountered in membrane cells is the production of chlorate in the anolyte compartment, which will readily not pass through a cation permselective membrane. Accordingly, chlorates concentrate in the anolyte, and after a brief period of operation may reach objectionable concentration levels. While chlorates are not known to rapidly deteriorate membrane or anode structures, high concentrations thereof reduce the concentration of electrolyte (salt) present, resulting in decreased efficiencies, possible chloride precipitation, and potentially adverse chlorate concentrations in caustic product.
In the past, removal of chlorate from diaphragm cell liquor has been handled in a number of ways. For example, Johnson, in U.S. Pat. No. 2,790,707, teaches removal of chlorates and chloride from diaphragm cell liquor by formation of complex iron salts by adding ferrous sulfate. Osborne, in U.S. Pat. No. 2,823,177, teaches prevention of chlorate formation during electrolysis of alkali metal chloride in diaphragm cells by destruction of hypochlorite through distribution of catalytic amounts of nickel or cobalt in the diaphragm. It is noteworthy that considerable effort has been expended in chlorate removal from cell liquor, a highly alkaline medium. In the presence of an excess of alkali, the chlorate is quite stable. It therefore tends to persist in the cell effluent and to pass on through to the evaporators in which the caustic alkali is concentrated. Practically all of the chlorate survives the evaporation and remains in the final product, where it constitutes a highly objectionable contaminant, especially to the Rayon industry.
The problem of lowering chlorates in diaphragm cells has been attacked at two main points:
(a) The chlorates having been formed, can be reduced in the further processing of the caustic alkali and by special treating methods. See for instance, U.S. Pat. Nos. 2,622,009; 2,044,888; 2,142,670; 2,207,595; 2,258,545; 2,403,789; 2,415,93, 2,446,868; and 2,562,169 which show representative examples of different methods used for reducing the chlorates after they have been formed; PA1 (b) The production of chlorates during the electrolysis can be lowered by adding a reagent to the brine feed which reacts preferentially with the back migrating hydroxyl ions from the cathode compartment of the cell making their way through the diaphragm into the anode compartment, and by such a reaction prevents the formation of some of the hypochlorites and thus additionally preventing these hypochlorites from further reacting to form chlorates. Reagents such as hydrochloric acid, shown in U.S. Pat. No. 585,330, and sulfur in an oxidizable form, such as sodium tetrasulfide, shown in U.S. Pat. No. 2,569,329 are illustrative of methods which have been used to attack the problem of chlorates in caustic by removing the back migrating hydroxyl ions before they can react to form chlorates.
In membrane cell operation, it is conventional to recycle spent brine from the anolyte compartment for resaturation. In the past, removal of chlorate from such recirculating brine has been accomplished by purging a portion of the stream and adding fresh brine as makeup. The purged chlorate-containing brine may, for example, be fed to a chlorate cell for use therein.